The invention generally relates to the production of polypropylene yarns and specifically to a method of making such yarns by melt spinning.
Melt spun polypropylene has been in commercial use for monofilaments, such as fishing lines, and staple fibers, such as carpet yarns. However, attempts to introduce polypropylene filament yarns into the apparel market have met with problems to the extent that quality fine denier yarns made of nylon or polyester are the rule while those made of propylene, if available at all, are the exception. Considering the lower costs of polypropylene as well as its unique properties, such as mechanical strength combined with thermal and chemical stability as well as its favorable ability to transfer moisture in the vapor phase, this is surprising since polypropylene would seem to provide for very desirable textile yarns.
The crucial problem, however, is that the processing technologies developed for polyesters and polyamides, notably the preoriented yarn (POY) methods, are not suitable at all for commercial polypropylene processing. This lacking transferability of established method and apparatus means for production of continuous yarns is believed to be due essentially to the fact that molten polypropylene behaves as a non-Newtonian liquid exhibiting structural viscosity phenomena that cause what is termed "draw resonance" or "spinning resonance" as illustrated, for example, in FIGS. 4 and 5 of EP--A--0 025 812 or US--A--4,347,206 incorporated herein by way of reference.
Briefly and in exaggeration, polypropylene not only exhibits dye-swelling upon extrusion but upon drawing-down from the swellings formed at the underside of the spinneret produces a filament with a non-uniform thickness in the manner of a string of linked sausages. Various prior art methods have been aimed either at modifying the polypropylene material or at specific methods (e.g. FR Patent No. 1,276,575, EP--A--0 028 844, DE--A--33 23 202) and it appears that acceptable results can be achieved best when semi-finished filament yarns are made in a first process by yarn producers and then textured and/or drawn to substantial orientation as required for most commercial uses of the yarns in a second separate process, e.g. by the yarn users.
However, integral methods, i.e. those starting from the unspun polymer and producing final polypropylene yarns composed of a plurality of continuous and substantially oriented filaments by melt spinning and stretching on a single production unit, have suffered either from low processing speeds of typically below 500 meters per minute or--when operable at acceptable production speeds of above 1000 meters per minute--from severe limitations as to the number of yarns that can be obtained per stretching installation unit. Consequently, production output per investment unit has not been satisfactory, or a multiplicity of stretching installation units had to be used and maintained.
Accordingly, it is a main object of the invention to provide for an integral method where a multitude of yarns, say 8 to 16 or more, can be obtained on a single stretching unit at speeds of above 1000 m/min yielding final product yarns that could either be in the form of fully oriented continuous yarns (FOY) and/or in the form of bulked continuous yarns (BCY) with yarn and filament deniers both for apparel use or any other yarn application where the unique properties of polypropylene provide an improved product.
A further object of the invention is an apparatus specially adapted for carrying out the novel method.